The provision of a broadcast service may require the transmission of one or more to of video, audio and other data signals. For example, a service may include a first stream carrying primary content, such as live or recorded music or a multimedia clip, and further streams carrying secondary content, such as error correction data, subtitles or song lyrics. Such services can be broadcast in a multiplexed transport stream using digital broadcasting networks, e.g. terrestrial Digital Video Broadcast (DVB-T), Integrated Services Digital Broadcasting (ISDB-T) or Advanced Television Systems Committee (ATSC-T), DVB-S (satellite), DVB-C (cable) or digital audio broadcasting (DAB) systems.
Wireless communication systems typically include one or more mobile terminals having stringent power requirements. Such a terminal may be required to operate for lengthy periods of time using only an internal source of power. In the case of simplex broadcast systems supporting unidirectional data delivery, e.g. DVB-T or DVB-S networks, a large proportion of the power consumption of the terminal arises from the demands of a receiver when receiving data. It is desirable to conserve power by reducing the amount of data received, i.e. by selectively receiving only data relating to service or content requested by the user.
Selective data reception for receiving a particular stream of data in a Time Division Multiple Access (TDMA) transmission can be implemented by switching the receiver between its on and off states, so that data reception is suspended during time slots relating to services or content that are not required. For example, in our co-pending application, GB0216240.2, a method is disclosed in which information conveyed in an IP service announcement is used to control the operation of a receiver in order to selectively receive a particular number of broadcast or multicast messages relating to that service. In another co-pending application, PCT/IB02/04823, a receiver is controlled so that data is selectively received according to a schedule of delivery time slots. The schedule is extracted from information relating to the content source as conveyed in a session announcement. When data reception is not required, i.e. when data relating to the selected service is not being transmitted, the receiver is disabled or switched into a low power operating mode in order to conserve power.
The performance of these methods may be improved by grouping related data packets into bursts before their transmission. The transmission takes the form of a sequence of bursts taking up most or all of the available bandwidth for a relatively short period of time. In this manner, each burst carries a significant amount of interrelated data. This further reduces the period of time for which the receiver is actively receiving data.
However, where data is broadcast to one or more mobile terminals in a cellular communication system, the same series of bursts may be transmitted to terminals located in first and second cells at different rates. FIG. 1 shows a series of bursts B1-B5 transmitted over two channels A, B. While each burst contains the same number P of data packets, the burst transmission rate of channel A exceeds that of channel B. Differences in the burst transmission rates may occur where, for example, the bit rate of channel A is greater than that of channel B or where the bit rate in one or both channels is shared between burst data and other data during the transmission of the bursts B1-B5, which results in the burst series being transmitted at different effective bit rates in channel A and channel B.
In FIG. 1, the duration of a burst B1 transmitted on channel B is longer than the duration of the corresponding burst on channel A. As the length of time required for the transmission of data on channel B is longer than that of an equivalent transmission on channel A, the interval between bursts B1, B2 in the series transmitted on channel B exceeds the interval between bursts transmitted on channel A. Therefore, the delay in the transmission of each burst in the series on channel B, when compared with channel A, increases over time. Problems may arise when a terminal moves from a location covered by a first cell, within which channel A is received, to another covered by a second cell, where the terminal receives the same data over channel B, as the increasing time offset between transmission of the corresponding bursts over channels A, B will disrupt selective reception of content or services in that terminal.